Friday 18 April 2014 08.49 EDT
First published on Friday 18 April 2014 08.49 EDT

Nelson Mandela was staring death in the face. Fifty years ago on Sunday, standing in the dock aware that he could be hanged for treason, the leader of South Africans' struggle against racial apartheid responded with one of the greatest speeches of the 20th century.

"During my lifetime I have dedicated myself to this struggle of the African people," he told the supreme court in Pretoria. "I have fought against white domination, and I have fought against black domination. I have cherished the ideal of a democratic and free society in which all persons live together in harmony and with equal opportunities. It is an ideal which I hope to live for and to achieve. But if needs be, it is an ideal for which I am prepared to die."

Today the courtroom in which Mandela – who would be sentenced to life imprisonment at the end of the Rivonia trial – spoke these words is still hearing cases beneath the austere grandeur of a stained-glass ceiling, ornate sconce lamps, a carved dark wood dais for the judge and a jury box with red leather seats that have been empty since South Africa scrapped juries in 1969.

Fourteen grey steps below the dock is a spartan concrete corridor leading to the holding cells where Mandela and his fellow accused were held. Chief among them is 5m x 7m room with walls coated in graffiti by generations of political prisoners. But after decades of neglect, the paint is cracking and peeling and in terminal decline, meaning this little-known historical treasure trove could soon be lost to the world.

The Freedom Charter at the Palace of Justice in Pretoria. Photograph: Nelson Mandela Foundation

Lawyer George Bizos, part of Mandela's defence team from 1963 to 1964, is leading calls to save it. "It does worry me," he said. "It's part of the historical event and part of our culture and certainly a site that should be preserved."

Bizos called for the cell to be recognised as a national heritage site, adding: "This is where we had consultations with Nelson Mandela, Walter Sisulu, Govan Mbeki and others. There should be respect to protect these things. I hope the people who have control take notice of it."

The cell has a bare concrete floor, one narrow barred window, a wide ventilation shaft against one wall, and the original heavy door with turn handle and peephole. The wall graffiti includes the yearned for freedom charter, a set of principles including "South Africa belongs to all who live in it", as well as phrases such as "Mandela no easy walk to freedom", "My dream is to be free, one love" and "Luthuli says the road to freedom is via the cross" a reference to former African National Congress (ANC) president Albert Luthuli.

There is also a description of an ANC "terrorist" trial in 1978 – "State closes the case. Defence starts its case" – and a list of prisoners who have come through the cell, including Tokyo Sexwale, later a government minister. There are drawings of unknown faces and a haunting image that depicts a stick man hanging from gallows; between 1961 and 1989, some 134 political prisoners were executed by the apartheid regime at Pretoria central prison.

The cell fell into disuse, and out of public consciousness, in the early 1990s. Since then, court facilities officer Chris Labuschagne estimates, only about 60 people have visited it, and the Guardian was the first this year. "It's part of history but you can see what's happening to the paint," he said. "They're supposed to put a Plexiglas screen over it. One day it will be gone and when it's gone, it's gone, and that will be sad."

Labuschagne, who has photographed the walls for posterity, said he would like the cell to be turned into a museum but added: "I don't think it's going to happen, not the way this paint has been damaged. There's not enough value given to history."

The cell lies in the bowels of the 19th-century Palace of Justice, a monument to colonial opulence incorporating British floor tiles, Dutch stained glass and wood from east India. A statue of former president Paul Kruger stands in the adjacent square. Three courts are still operating, including court C, which hosted what remains the most significant political trial in South African history – one that puts the circus around Oscar Pistorius, a short walk over the road, into some kind of perspective.

The 50th anniversary of the late Mandela's "speech from the dock" will be marked at the former Liliesleaf farm in northern Johannesburg on Sunday with a performance by musician Sipho "Hotstix" Mabuse. Nicholas Wolpe, chief executive officer of the Liliesleaf Trust, said the holding cell must not be allowed to decay.

"The fact that we are allowing a revealing aspect of a key moment in our history to disintegrate and fade away goes to the core of our battle to keep our history alive and real," he said. "It is indicative of our ambivalence and attitude towards our liberation struggle and our history in general.

"That is why commemorating events like the 50th anniversary of the raid on Liliesleaf and Nelson Mandela's 'I am prepared to die,' statement from the dock is essential in our struggle to keep not only the memory of our history alive, but also our understanding."